1. Field of the Invention
This invention generally relates to a pulse-width modulation device and more particularly to a pulse-width modulation device for controlling laser beams emitted by a laser provided in a laser-beam printer.
2. Description of the Prior Art
There has been developed a practical laser-beam printer by combining a laser with an electrophotographic system. In case of such a laser-beam printer, for the purpose of printing a halftone image, a pulse-width modulation of a laser-light modulation signal is performed to control a printing time required for printing data corresponding to each pixel.
FIG. 8 is a schematic block diagram for illustrating the configuration of a conventional pulse-width modulation device. FIG. 9 is a timing chart for illustrating an operation of the device of FIG. 8.
Hereinafter, the configuration and operation of the device of FIG. 8 will be described by referring to FIGS. 8 and 9. In FIG. 8, reference character ff represents reference clock signal; and 51 a timing matching circuit. When, for example, a signal representing 3-bit input data DD is input to the timing matching circuit 51 in synchronization with an input clock signal ee, the timing matching circuit 51 converts the input data DD into predetermined type data AA and then outputs the data AA to a comparator 54. Reference numeral 52 denotes a code converter which converts a value CC counted by a counter 53 into data represented by a signal BB and subsequently outputs the signal BB to the comparator 54. Further, reference character gg designates a clearing signal. The value CC held in the counter 53 is cleared in synchronization with a leading edge of the clearing signal gg. Further, the comparator 54 first compares the data AA with the value (hereunder referred to simply as the data BB) indicated by the signal BB. Subsequently, the comparator 54 outputs a result-of-comparison signal HH, which represents a result of the comparison and has a high voltage level in case where the data AA is greater than the data BB, to a device (not shown) for performing what is called a glitch deleting processing. After such a processing is performed on the result-of-comparison signal HH, the signal HH is input to a pulse generating circuit 55 which then outputs a pulse signal II as illustrated in FIG. 9.
FIG. 10 is a diagram for illustrating a pulse-width modulation operation performed on the pulse signal II output from the pulse generating circuit 55 of FIG. 8.
As is seen from FIGS. 9 and 10, the position of a dot is put in the vicinity of a center of a corresponding one of the pulses II in case of the conventional pulse-width modulation device of FIG. 8.
Hereinafter, problems, which occur in case where an image is output from a laser-beam printer employing the conventional pulse-width modulation device of FIG. 8, will be described by referring to FIG. 11.
FIG. 11 is a diagram for illustrating an example of a dot output or printed by the laser-beam printer by employing the device of FIG. 8 in case of using a 3.times.3 threshold-value matrix as mapped input image data.
FIG. 11(a) shows the contents of the 3.times.3 threshold-value matrix. As shown in this figure, there are what is called a micro-pixel (hereunder referred to simply as a pixel) having a gray level (or tone level) of 7 and an adjoining pixel having a tone level of 4 in the matrix. FIG. 11(b) shows a pulse-width modulation signal produced in this case. As shown in this figure, there are two discontinuous pulses. Further, FIG. 11(c) shows a corresponding energy distribution in this case. Moreover, FIG. 11(d) shows a dot reproduced by the laser-beam printer as a result of using the device of FIG. 8. As is seen from FIG. 11(d), there is only a small joint between dots respectively corresponding to the pixels, the gray levels of which are 7 and 4, respectively. Namely, connection between the dots respectively corresponding to these adjoining pixels is bad. Note that this is due to discontinuity of the pulse-width modulation signal (namely, due to the presence of the discontinuous pulses) as illustrated in FIG. 11(b). Further, in some cases, there is no joint between dots respectively corresponding to two adjacent pixels. Namely, in some cases, dots respectively corresponding to two adjacent pixels are discontinuous. Thus the spatial frequency of a dot pattern becomes high. Consequently, the conventional pulse-width modulation device has a drawback in that no "stable" halftone image can be output. Moreover, the conventional pulse-width modulation device has other drawbacks in that the circuit configuration thereof is complex and in that the cost thereof is high. The present invention is accomplished to eliminate the drawbacks of the conventional pulse-width modulation device.
It is, accordingly, an object of the present invention to provide a pulse-width modulation device which has a simple circuit configuration and can obtain a "stable" halftone image (namely, a halftone image of good picture-quality and high reproducibility) by preventing an occurrence of discontinuous pulses of a pulse-width modulation signal.